Many electronic devices, such as mobile phones, include a microphone. The microphone may be, for example, a near-speech microphone for detecting voice conversation during a telephone call. Additional microphones may be included on an electronic device for detecting environmental sounds. For example, a reference microphone may be included in an electronic device to measure background noise provided as feedback to a noise cancelling algorithm.
Performance of conventional microphones is, at least in part, proportional to microphone area and pre-amplifier power. For example, increasing the area of a microphone by four times and increasing the pre-amp power by four times may result in a dynamic range improved by two times. However, the cost of a microphone increases rapidly with microphone area because larger microphones are more fragile and difficult to manufacture. Increasing costs of microphones to obtain increased performance may not be well tolerated in certain devices, such as certain types of mobile phones. Instead, higher cost microphones are often limited to specific markets, such as audio recording.
One solution for performance issues is to implement an array of microphones to obtain better audio input. Arrays of microphones have been implemented in specific markets that are relatively cost-insensitive, such as when recording surround sound audio for movies. However, building multiple microphones into a consumer electronic device has typically been cost-prohibitive.
Conventional microphones have limitations that prevent large arrays of microphones from being constructed cost-effectively in an electronic device. For example, analog signals are incapable of traveling long distances without degradation of the analog signal. Additionally, when an array of microphones is constructed, the number of wires between the array of microphones and a head-end chip increases proportional to the number of microphones in the array. The head-end chip may be a processor, such as a digital signal processor (DSP), located between the array of microphones and other circuitry in an electronic device. That is, if each microphone has a three-wire connection, then an array of ten microphones may have a total of as many as thirty wires. A large number of wire connections complicates layout of a circuit board for interconnecting the head-end chip with the array of microphones.
Furthermore, microphones have limited noise rejection capability. For example, digital microphones (DMICs) often have noise rejection of about 20 decibels. Low noise rejection in a microphone leaves the microphone susceptible to degraded performance from a noisy power supply. Conventionally, a reference voltage in an electronic device powering the microphone has a high noise density resulting from amplifying a small proportional-to-absolute-temperature (PTAT) voltage to add to a transistor voltage drop, VBE, to produce the reference voltage. An amplifier delivering the reference voltage may add noise as much as 100 nV/√{square root over (Hz)}. One prior solution is to co-locate a large capacitor with each microphone to limit the bandwidth of power supply noise received at the microphone. However, high capacitance capacitors consume a large amount of circuit board space.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved microphones or microphone array technology, particularly for consumer-level devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.